Silver contact



Patented June 6, 1939 SILVER CONTACT Franz R. Hensel and Kenneth L. Emmert, Indianapolis, Ind., assignors to P. R. Mallory & Co., Inc., Indianapolis, Ind., a corporation of Delaware N Drawing.

Application August 6, 1938,

Serial No. 223,506

4 Claims.

This invention relates to electric contacts.

An object of the invention is to improve the characteristics ofthe electric contact elements containing silver.

Another object of the invention is-to provide a silver base contact material which will retain a very low contact resistance.

A still further object is to provide a new contact material which can be used under severe electrical loads without welding or sticking.

It is a further object of the invention to provide a contact capable of operating on high frequency at heavy current values, Without objectionable metal transfer.

Other objects of the invention will be apparent from the following description taken in connection with the appended claims.

The present invention comprises the combination of elements, methods of manufacture and the products thereof, brought out and exemplified in the disclosure hereinafter set forth, the scope of the invention being indicated in the appended claims.

the invention.

The present invention comprises an improvement in silver base electric contact elements and especially in contact elements, formed of at least 70% silver. According to the invention, the contacts are improved by the addition of magnesium.

The contacts may be composed of silver and magnesium accordingly. The ingredients may be present in approximately the following proportions: Magnesium per cent .05-20 Balance Predominantly silver The preferred range of magnesium is .l-8%. Up to that percentage, the magnesium is taken up in alpha solution in silver.

con, titanium, zirconium,

substantial proportions indicated above.

We have found that silver and magnesium, up to 8% magnesium, can be alloyed very readily and the cast ingots can be easily swaged and drawn into the form of wires or rolled into the form of sheet, from which contacts may be prepared by a punching operation, screw machine operation or by cold heading.

In an exhaustive study of silver base contact materials, a number of tests were devised to study their suitability for electrical contacting 0 purposes.

A comparison test was conducted, wherein contacts of similar physical dimensions were tested on a resistive inductive circuit at 470 cycles per minute and wherein the current flowing in the circuit was increased periodically to obtain definite current values of alloys in the nature de scribed above, in comparison with contact materials produced in the prior art. The amount of material transfer from one contact to the other was also used as a method of comparison.

In order to present comparative figures, values are given for alloys of silver, containing 5% by weight of the materials listed.

Height of Contact material g gg material transferred Amperes Fine silver 12 .010 Silver plus 5% copper 12 .011 Silver plus 5% zinc 10 .014 Silver plus 5% cadmium. 13 .028 Silver plus 5% molybdenum 12 062 Silver plus 5% manganese 8 007 Silver plus 5% nickel 12 .028 Silver plus 5% tungsten 13 056 Silver plus 5% magnesium 17 002 From the above table, the outstanding performance of our new contact alloy is very evident. Furthermore, the small transfer is so uniform in the present alloy and it does not occur in the form of needle points, so that the material can be used for extremely sensitive relay applications as well as for heavy duty applications. This is also indicated by measurements 5 which were made on the contact resistance of this new alloy combination. Experiments consisted in a test of 1000 operations on a relay with grams contact pressure, 1330 operations per hour, with a resistive load. The contact resist- 50 ance at 110 D. C., 1.9 amp. current, was 2.06 milliohms. After another 5000 operations with the same load, the contact resistance was 2.73 milliohms. At 110 volt A. C., and 6.5 amp. current, the contact resistance after 1000 operations, was 2.08, and after 5000 operations, was 2.27 milliohms. At 220 volt A. C., and 10 amps, the contact resistance after 1000 operations was 1.71 milliohms and after 5000 operations, 2.13 milliohms. The above results show that ex tremely low values of contact resistance are retained over long periods of operation. For a comparison, contact resistance values are given on alloys of the prior art. Standard coin silver will have a contact resistance of 10 milliohms,v

when tested under the same conditions at 110 volt D. C. and 1.9 amps.

An alloy of silver and. 5% molybdenum, showed a contact resistance of 27 milliohms under the same conditions.

The contact resistance of the present alloy is as low as that of fine silver, or, even lower. The alloy has the definite advantage that the-contact resistance values are not erratic, but very con stant. Very often it is experienced that in service a high contact resistance will be built up after a shortperiod of operation; and if the contacts continue in operation, the contact resistance might drop down again to lower values and vary as much as 10 milliohms with fine silver.

These three most outstanding values pointed out by the above investigations, namely, high limiting current, extremely low transfer and extremely low contact resistance, make this new material superior to almost any known silver base alloy-of the prior art.

In addition it should be pointed out that 5 weight percent of magnesium amounts to as much as 30 volume per cent of magnesium. The

r specific gravity of the finished alloy is therefore considerably reduced for a 5% alloy and amounts to approximately 8.39. This factor will contribute materially to obtain a considerable reduction of price in the finished contact, because a larger number of contacts can be produced from a unit weight of contact wire and contact sheet.

The alloy has an'electrical conductivity which is high enough to lee-satisfactory for electrical contacting purposes. A,.5% alloy has an electrical conductivity of atleast 20% (I. A. C. S.). In contact applications the specific resistance is considerably less; important than the contact resistance, as long as it is high enough to carry the current satisfactorily. The thermal conductivity of the new alloy is also quite high and therefore will prevent over-heating ofthe contacts. The alloys can be hardened by a com bination of suitable heat-treating and cold working. The alloys are ductile enough so that they can be very easily heated and will flow readily in the cold working operations.

The fact that magnesium has a boiling point which is about 1100 degrees C., and which is considerably lower than the boiling point of silver, which is in the neighborhood of 1950 C., is also a contributing factor for the excellent performance of the new contact alloy combination.

The silver-magnesium alloys can readily be prepared in the form of contact bi-metals in which one side consists of a base metal, such as an alloy of iron, nickel or a copper base material of silver-magnesium on the other side. The alloy lends itself readily to the formation of an electrical contact by. a. beading or fusing the alloy directly to the surface of the base metal blank, suchas a rivet, screw or disc.

When operated on voltage regulators, such as used in the regulation of automotive generators, the material of the present invention, maintained perfect regulation. After continuous operations, exceeding many hundred hours, the resistance value of the new alloy was considerably below that normally encountered on alloys previously adopted for this type ofv service.

While the present invention as to its objects and advantages, has been described herein, as carriedout in specificembodiments thereof, it is not desired to be limited thereby, but it is intended to cover the invention broadly, within the spirit and scope of the appended claims.

What is claimed is:

1. An electrical contacting member formed of an alloy containing as an essentialv ingredient, silver to which has been added magnesium, in quantities ranging from .05 to 20%.

2. An electrical contact member formedof an alloy of .1 to 8% magnesium' and balance substantially all silver.

3. An electric contact formed of an alloy of .1 to 8% magnesium, balance silver.

4. An electric contact formed of a silver base alloy containing .1 to 20% magnesium, said contact being. characterized by low contact resistance and the ability to retain. said low resistance throughout long periods of operation, low tendency toward metal transfer and relatively great freedom from welding or sticking during operation on high frequency or at heavy current values.

FRANZ R. HENSEL. KENNETH L. EMMERT. 

